A commonly used principle for transmitting data over a radio channel and for overcoming the signal rate limitation of binary sequence signalling is to make use of four or more unique symbols. Thereby, the bit rate can exceed the maximum signal rate (in bits/s) corresponding to double the pass-band (in Hz) as given by the Nyquist theorem.
Quadrature phase shift keying (QPSK) also denoted 4-state quadrature amplitude modulation (4-QAM) involves that two-bit words are coded into four discrete symbols. These symbols can be represented as signal vectors in the complex plane having constant amplitude but four distinct phase values in relation to a reference signal. Detection is carried out by establishing which of four quadrants in the complex plane the received signal can be referred to.
If a higher modulation order is used, the bit rate can be increased further. However, higher requirements are inflicted on the detection stage since it becomes more difficult to detect the individual symbols from one another, as they appear closer in the complex plane. The deterioration of the signal as transmitted over a given media also constitutes a limitation to the possible number of symbols being used.
Higher order keying is commonly referred to as M'ary QAM, where M=2N refers to the number of discrete symbols being available, whereby N bits can be transmitted per symbol. M'ary QAM is also referred to as M'ary APK (amplitude phase shift keying), as both the amplitude and phase may vary for individual symbols.
FIG. 1 shows a conventional transmitter and FIG. 2 shows a conventional receiver.
The transmitter unit comprises a data buffer 1, a mapper 2, baseband filtering unit 3, intermediate frequency (IF) oscillator 6, phase divider 5, adders 7, and summer 4 from which a radio frequency (RF) signal is transmitted.
Data stored temporarily in buffer 1 is conveyed to the mapper 2 in accordance with the rate data can be transmitted over the radio interface. The data, which can be seen as a binary bit serial string, is partitioned into symbols by the mapper 2 having an I component and Q component in the complex plane as explained above.
The receiver, on the other hand, decodes I and Q components multiplying the incoming signal (RF) with 90 degree phase skewed signals provided by signal oscillator IF12 from divider D11. The signal of IF 12 is typically rendered coherent by means of a carrier recovery PLL (phase locked loop) with the carrier signal from IF 6, such that the RF signal, after being filtered in respective filters 9 and 10, can be decoded back into the complex plane. An error signal E corresponding to the deviation of the detected symbol value from an expected symbol value is fed into PLL loop back filter 13 adjusting IF generator IF 12.
FIGS. 3 and 4 show a conventional scheme for transmitting data. A frame alignment word F1 consisting of a predetermined sequence of symbols functions as a reference for subsequent frames of traffic data B1, B2 . . . BN−1. For example, the frame-word may have a length of 8 bits. After transmission of a fixed period of frames, the frame alignment word is repeated. Via a frame-aligner 15, in which the predetermined sequence is recovered, the demodulator, can identify the individual frame position for each frame.
As shown in FIG. 4′, each frame following the frame alignment word may include a pilot signal.
As is shown in FIG. 4″, the frame alignment word may comprise a single pilot signal, which is discernible from the remaining symbols.
In known systems, the frame alignment word (or pilot signal) may for instance appear for every 20.000 symbols.
Since pilot signals are associated with “spectral peaks”, and therefore may disturb other channels or systems, the frequency at which pilot signals occur is normally restricted.
By definition, frame alignment words and pilot signals do not contain any traffic information (payload) and are therefore regarded as an overhead to the information being transmitted.
Forward error correction (FEC) methods can advantageously be used to restore the signal content where the signal to noise ratio impairs the signal. However, as is known, FEC implies using some redundancy or overhead to the information being transmitted.
Prior art document EP1022874 discloses an apparatus for digital data transmission, utilising for instance M'ary QAM. A frame configuration is used in which a pilot symbol, i.e. a signal carrying no traffic data, is inserted for every N−1 information symbols. The receiver estimates the phase, amplitude variation and frequency offset on the I-Q plane from the pilot symbol, by inserting for instance three pilot signals in a row. In order to increase transmission efficiency, symbols immediately before and after a single pilot symbol are modulated according to a modulation type different from the pilot modulation type. A method is shown for differentiating the modulation type for modulating pilot symbols from the modulation type for modulating symbols immediately before and after a pilot symbol, which includes placing two or more signal points of each one symbol immediately before and after a pilot symbol on a virtual line connecting the pilot signal point and the origin in the in-phase I—quadrature Q plane. According to one embodiment of the above document (FIG. 13), the pilot symbol and the symbols immediately before and after the pilot symbol coincide with symbols of a 64-QAM-modulation scheme used for data transmission.